The present invention relates to a non single crystal semiconductor device, in particular, relates to a thin film transistor (TFT) which has a substrate made of glass, and a thin film made of non single crystal silicon deposited on the substrate.
A thin film transistor (TFT) has been used in an image sensor used in a facsimile device, and/or a liquid crystal display panel. A thin film transistor is produced in a non single crystal semiconductor film, which is polycrystal silicon layer deposited on a substrate of quartz or glass.
When a substrate is made of quartz, a transistor is produced on the substrate through high temperature process up to 900.degree. C.
However, a quartz substrate has the disadvantage that the price of the substrate is high, and the high price of the substrate increases the price of a semiconductor device itself.
When a substrate is glass, a transistor is produced on the substrate through relatively low temperature up to 600.degree. C., because glass is softened or melted if temperature is higher than 600.degree. C. In this case, a non single crystal film is produced on a glass substrate through low temperature process up to 600.degree. C.
Conventionally, a non single crystal film, or an active layer of a MOS transistor, is produced through the steps of growing amorphous silicon layer on a substrate through plasma CVD process or low pressure CVD process (LPCVD) which uses silane gas (SiH.sub.4), and effecting solid phase growth to said amorphous film.
As for solid phase growth, a long time aneal process, and a laser aneal process have been known.
In a long time aneal process, an amorphous film is heated at temperature 550.degree. C.-600.degree. C. in nitrogen gas atmosphere for 8-56 hours so that an amorphous film is crystalized. A MOSFET transistor produced in an active layer produced through said long time aneal process has the mobility up to 35 cm.sup.2 /V.multidot.sec in an N-channel. But the mobility in P-channel is only 10 cm.sup.2 /V.multidot.sec. Generally speaking, the higher the mobility is, the higher the switching speed of a transistor is, and it is desirous that the mobility is higher than 25 cm.sup.2 /V.multidot.sec. Further, it has the disadvantage that the threshold voltage of the transistor is about 17 V, which is undesirably quite high. The threshold voltage is defined as gate voltage which switches a transistor, and the preferable threshold voltage is less than 10 Volts. Therefore, the long time aneal process is not suitable both in the mobility, and the threshold voltage.
In a laser aneal process, an amorphous film is crystalized by illuminating and heating the same with a laser beam spot. The mobility in an active layer produced through the laser aneal process is quite high up to 100 cm.sup.2 /V.multidot.sec in both P-channel and N-channel. However, the laser aneal process has the disadvantage that it is difficult to produce a large active film for a large display screen or a large line sensor, since a laser beam is a small spot, and it is impossible to heat the amorphous film uniformly.